1. Field of the Invention
This application is related to the field of switching power regulators for supplying output current to a pulsed load.
2. Description of the Prior Art
Switched mode power supplies for converting DC voltages in inductive-capacitive discharge pulsed radar applications are known in the prior art. Often they employ a buck-derived topology with feedback loops for voltage regulation and over current protection. Switched mode processes inherently produce ripple voltages due to the spurious frequencies generated when the DC voltages are switched into the reactive components. The ripple voltage limits the use of switched mode power converters in many applications. For example, ripple atop the output pulses supplied by power converters to RF transmitters interfere with signal processing of radar returns by reducing the range and or target detectability of the return signal.
The buck mode converter provides a steady state output current with a superimposed saw tooth shaped ripple pattern. Filter capacitors at the output supply attenuate the ripple current to provide a low peak-to-peak voltage ripple at the load. This low-level ripple may be excessive for noise intolerant loads. For example, radar RF amplifiers are sensitive to low-level ripple, which affect the output amplitude of the RF pulse burst having a direct effect on target discrimination and ranging capability. To reduce the problem, the prior art adds one or more stages of filtering using series connected inductors and shunt connected capacitors.
In LC filter circuits it is well known that inductor impedance increases with increasing frequency while the capacitor impedance decreases with increasing frequency. When the inductors are in series with the current supply and the capacitors return to ground, these components provide a low pass filter operation that lowers the high frequency ripple generated by the switched mode power converter without dissipating additional excessive power. However, each inductor and capacitor employed also contains additional electrical components referred to as parasitics in their realizable implementations. An inductor constructed from wire wrapped around a form creates multiple capacitors in parallel with the inductance. These tend to reduce the impedance of the inductor at higher filtering frequencies. The wire itself contributes additional series resistance. The capacitors contain series resistance to current flow due to both the dielectric material used in their construction and the physical connections from the external leads to the internal plates of the capacitor. At the higher power levels found in radar pulsed loads, it is common practice to use aluminum or tantalum type capacitors for both filtering and energy storage. Both types have a relatively large equivalent series resistance, which reduces the ripple noise attenuation achievable from the filter capacitance and inductance alone.
Adding filters increases the weight and volume of a product. The extra filtering also creates additional delays in the re-supply of power to the output capacitors. This slows the supply's response to transient pulse loads and introduces phase shifts to the feedback path, which are detrimental to voltage and current regulation. Any additional filtering should supply as much attenuation of the unwanted signals as possible without introducing excessive phase shifts that would complicate closed loop controller operations. Additionally, any added filtering should consume as little of the available volume as possible so that it can fit within the design envelope.